This invention relates to filtering networks useful for electric power conversion systems where multiphase alternating current power is converted to direct current power and vice versa. Such techniques are used, for example, for direct current power transmission, which is becoming more popular because with newly developed transmission techniques, as in those using supercooled conductors, power may be transmitted more efficiently with fewer losses. However, there is at present no widely used technology to directly produce DC power, and it is necessary to convert AC power to DC power and then back to AC power for transmission by DC techniques. AC to DC power converters generally rectify multiphase AC with a controlled rectifier bridge as in the Graetz type converter, so as to produce DC power. Such conversion techniques produce undesirable harmonic currents of frequencies which are harmonics of the fundamental or base frequency of the AC power. Because these harmonic currents contribute to losses in the power system and because some of them tend to interfere with communication equipment, it is necessary to provide filtering networks in conjunction with the conversion devices to eliminate these undesirable harmonic currents. For example, with 60 Hz-3 phase AC power converted to DC by a twelve pulse Graetz bridge type converter, problems with 11th (660 Hz) and 13th (780 Hz) harmonics are particularly acute.